By Terry Malone, PE, SE, WoodWorks – Wood Products Council
Nationwide, there has been an increase in the demand for multi-story mixed-use and multi-residential structures. Common configurations include up to five stories of residential use over retail, commercial, office and/or parking occupancies, similar in configuration to the building shown in Figure 1. Podium designs are one way to maximize the number of stories, increase unit density and lower construction costs. This article covers important design considerations and traditional approaches related to the design of a five-story wood-framed structure over a two-story concrete or masonry podium.
The 2015 International Building Code (IBC) allows a maximum building height above grade of 75 feet using Type IIIB construction and 85 feet for Type IIIA if NFPA 13 sprinklers are used. However, they only allow up to five stories for Type IIIA or IIIB structures under those same conditions. Structural provisions in the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7) limit the maximum height of wood structural panel-sheathed shear walls to 65 feet above the base of the seismic force-resisting system (SFRS) in Seismic Design Category (SDC) D, E, or F. To gain additional stories, increase building area, and stay within the allowable building and seismic system heights, the IBC and ASCE 7 each have provisions which enable podium designs.
IBC 2015 Section 510.2 allows an upper portion of any construction type to be built over a lower portion where the two portions are treated as separate and distinct structures. This is for purposes of determining the allowable area limitation, continuity of fire walls, type of construction, and number of stories. This allowance only applies when:
- The building portions are separated by a horizontal assembly with a minimum 3-hour fire-resistance rating,
- The building below is of Type IA Construction and is protected throughout with NFPA 13 sprinklers,
- Shafts, stairways, ramps, and escalator enclosures penetrating the horizontal assembly have a 2-hour fire-resistance rating, and
- The maximum building height measured in feet above grade is not exceeded.
The 2015 IBC allows multiple-story podiums. This allows two stories of podium with five stories of wood framing above to meet the 85-foot maximum building height limitation and also meet the 65-foot SFRS height limit.
Example floor plan configurations typically encountered in mid-rise multi-family construction are shown in Figure 2. These plans are frequently rectangular, with or without exterior shear walls, or they can have multiple horizontal offsets and wings. The lateral force-resisting system for the flexible upper portion is typically light-framed shear walls sheathed with wood structural panels (WSP). Many if not all of the walls separating the dwelling units are used as interior shear walls in the transverse direction. Lateral forces in the longitudinal direction are typically resisted by the exterior walls and corridor walls. If a rigid diaphragm analysis is warranted, the transverse walls would also act to resist torsional forces.
Designers of these buildings should avoid having more than one SFRS in the flexible upper portion. ASCE 7-10 Section 12.2.3 notes that when combining different seismic force-resisting systems in the same direction, the most stringent applicable structural system limitations of ASCE 7-10 Table 12.2-1 shall apply. For example, light-framed shear walls with WSP sheathing have a response modification coefficient of R=6.5. Combining light-framed shear walls sheathed with other materials (e.g., gypsum wallboard) having a response modification coefficient of R=2, would require the WSP walls to be designed for forces in excess of three times greater (6.5/2) than if only WSP walls are used. Similar force modifications for wind demands do not apply.
Two-Stage Seismic Analysis
Structurally, ASCE 7-10 Section 184.108.40.206 provides a two-stage analysis procedure that can be beneficial for seismic design of podium projects. The procedure treats the flexible upper and rigid lower portion portions of the structure as two distinct structures, thereby simplifying the seismic design process. Only the weight of the flexible upper portion has to be considered in its design, not the entire weight of both portions. The two-stage analysis also allows the seismic base of the upper portion to be the top of the lower portion. This allows measuring the maximum SFRS height for a wood structural panel-sheathed shear wall system in SDC D through F of 65 feet, from the top of the podium. The requirements for a two-stage analysis are:
a. The stiffness of the lower section is ten times the stiffness of the upper section.
b. The period of the entire structure is not more than 1.1 times the period of the upper portion considered as a separate structure supported at the transition from the upper to lower portions.
c. The upper portion is designed as a separate structure using the appropriate R and redundancy factor, ρ.
d. The lower portion is designed as a separate structure using the appropriate R and ρ. The reactions from the upper portion are determined from the analysis of the upper portion amplified by the ratio of the R/ ρ of the upper portion over the R/ ρ of the lower portion. This ratio is not less than 1.0.
e. The upper portion is analyzed with the equivalent lateral force or modal response spectrum procedure, and the lower portion is analyzed with the equivalent lateral force procedure.
Some confusion exists regarding the required amplification of the forces that are transferred from the flexible upper portion into the podium slab. The amplification factor in 220.127.116.11 (d), when used, applies to only the seismic component of the reaction forces, not the entire reaction-included gravity loads. Gravity framing (e.g., beam, post-tensioned slabs, columns) supporting a discontinuous shear wall is designed for overstrength where required by ASCE 7-10 Section 18.104.22.168. Connection requirements to the podium slab are as shown in Figure 3.
Distribution of forces to the vertical-resisting elements are based on analysis methods where the diaphragm is modeled as follows:
- Idealized as flexible – The distribution is based on tributary area. In common multi-family shear wall layouts, this can underestimate forces distributed to the corridor walls and overestimate forces distributed to the exterior walls with a similar impact on diaphragm forces being delivered to the walls.
- Idealized as rigid – The distribution is based on relative lateral stiffnesses of vertical-resisting elements of the story below. This more conservatively distributes lateral forces to corridor and transverse walls and allows easier determination of building drift, but can overestimate torsional drift and underestimate forces distributed to the exterior walls, including diaphragm forces.
- Modelled as semi-rigid – The diaphragm is not idealized as rigid or flexible. Shear is distributed to the vertical-resisting elements based on the relative stiffnesses of the diaphragm and the vertical-resisting elements, accounting for both shear and flexural deformations. In lieu of a semi-rigid diaphragm analysis, it is permitted in the American Wood Council’s Special Design Provisions for Wind and Seismic (SDPWS) 2015 Section 4.2.5 to use an enveloped analysis, analyzing for both flexible and rigid conditions and taking the largest forces.
Current practice for light-frame construction commonly assumes that wood diaphragms are flexible for the purpose of distributing horizontal forces to shear walls. ASCE 7-10 Section 22.214.171.124 (c) allows diaphragms in light-frame structures to be idealized as flexible when 1 ½ inches or less of non-structural topping, such as concrete or a similar material, is placed over WSP diaphragms, and each line of vertical elements of the SFRS complies with the allowable story drift of ASCE 7-10 Table 12.12-1. Using the flexible diaphragm assumption would allow the distribution of diaphragm forces to the shear walls to be based on tributary area. In 1999, the Structural Engineers Association of California code and seismology committees recommended that relative flexibility requirements outlined in ASCE 7 Section 12.3.1 be considered for wood-framed diaphragms.
12.3.1 Diaphragm Flexibility
The structural analysis shall consider the relative stiffnesses of the diaphragms and of the vertical elements of the seismic force-resisting system. Unless a diaphragm can be idealized as either flexible or rigid in accordance with Sections 126.96.36.199, 188.8.131.52 or 184.108.40.206, the structural analysis shall explicitly include consideration of the stiffness of the diaphragm (i.e., semi-rigid modelling assumption).
Even though diaphragms may be idealized as flexible, it is sometimes good engineering judgement to consider other flexibility conditions. Currently, some designers only perform a flexible diaphragm analysis, and some a rigid diaphragm analysis, but a few use semi-rigid modeling (enveloping). On that basis, some confusion and lack of consistency exists regarding which type of diaphragm analysis should be employed for a given project. Verifying the diaphragm flexibility is becoming increasingly more important given trends toward larger openings in exterior shear walls, shorter wall lengths, and a greater number of wood-frame stories over the podium.
Mid-rise structures using podium designs provide many opportunities for cost-effective, higher-density construction. It has become increasingly important to consider the relative stiffness of the diaphragms and shear walls, and the effects of multi-story shear walls as buildings become taller and more complex in shape. Research, full-scale testing and performance-based studies continue to evolve, which impact both changes to the building code and guidelines for engineers. Recognized, comprehensive guidelines and design examples providing in-depth coverage are available demonstrating traditional methods of analyzing mid-rise and podium designs.
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